Long step out direct electric heating assembly

ABSTRACT

A subsea direct electrical heating assembly adapted to heat a hydrocarbon conducting steel pipeline ( 1 ) arranged subsea. The assembly comprises a direct electrical heating cable ( 3 ) extending along and being connected to the steel pipeline ( 1 ) and a power transmission cable ( 7 ) receiving electric power from a power supply ( 5 ) which is arranged onshore or at surface offshore, and which feeds the direct electrical heating cable ( 3 ). The subsea direct electrical heating assembly comprises a power conditioning arrangement ( 100 ) arranged at a subsea location, in a position between the power transmission cable ( 7 ) and the direct electrical heating cable ( 3 ). The power transmission cable ( 7 ) extends from the offshore or onshore power supply ( 5 ) and down to the power conditioning arrangement ( 100 ).

The present invention relates to heating of long subsea flowlinesconducting hydrocarbons and long distance power supply via subseacables. In particular it relates to the method known in the art asdirect electric heating, wherein electric power is used to heat thepipelines in order to prevent hydrate formation. The assembly isparticularly suitable for hydrate/wax prevention in long step-outflowlines by direct electric heating of e.g. 10″ to 30″ carbon pipelinesin the range of e.g. 60-300 km or more.

BACKGROUND

Direct electric heating (DEH) of long flowlines and large exportpipelines provides many advantages compared to alternative methods. DEHhas been applied actively in the past 10 years to prevent hydrateformation and is now breaking new ground that was not previously beingconsidered feasible. By using qualified technology and existing designmodels, longer and larger pipelines can be heated intermittently orcontinuously.

Direct Electrical Heated Pipe in Pipe (DEHPIP) is a slightly differenttechnological approach to the same problem that have quite similardemands for the electrical power supply system to drive them, hence mostof the electrical energy supply system topologies can be used to powerboth DEH and DEHPIP systems independent of end-fed or midpoint fedtopologies. Common for both systems is that the electric current flowsaxially through the pipe wall causing direct heating of the pipeline.

Wet-insulated: Open Loop System

-   -   End-Fed Pipe    -   Center-Fed Pipe

Dry-insulated: Closed Loop System

-   -   End-Fed Pipe-in-Pipe    -   Center-Fed Pipe-in-Pipe

DEHPIP systems are sometimes described as Electrical Flowline Heating(EFH) systems since EFH systems traditionally have been associated withthe Dry Insulated (Pipe-in-Pipe) flowline heating system technology, butthe term can also be used as a general reference to any flowline heatingusing electricity.

Electric Heating of Pipelines is attractive for short and long step outsas DEH operating costs are considerably reduced compared to the use ofchemicals. The technology is unique and commercially and technicallyattractive. It allows for the use of DEH for both infield flowlines,tie-backs and export pipelines with diameters around 6″ to 30″ andabove. An increased number of DEH assemblies has been evaluated for oiland gas fields or project developments concepts around the world and theextension of this new technology will generally give higher flexibilityin operation of the fields during planned or unplanned shut downs.Material aging and other failure mechanisms caused by high temperaturesand water pressure are also of great importance. Accuracy in design andanalysis as well as industry experience are important in solving projectspecific hydrate or wax issues in long DEH systems.

Using DEH can involve arranging a DEH cable along a steel pipeline.Current is guided through the DEH cable in one direction and returnedthrough the pipeline steel in the return direction. Heat is generated inthe pipeline steel, partly due to ohmic resistance in the steel andpartly due to induced heat, as the current is an alternating current. Asthe contact between the DEH cable and the pipeline steel is notinsulated from the surrounding sea water, a fraction of the current willalso flow through the sea water and not in the pipeline.

Patent application publication EP2166637 (Siemens Aktiengesellschaft)describes a power supply arrangement for direct electrical heating (DEH)of a pipeline system. The power supply arrangement has a three phasetransformer and a compensation unit including a capacitor means, and isadapted to feed electrical power to a single phase load.

WO2007011230 (Aker Kværner Engineering & Technology) describes a systemfor power supply to a flowline heating circuit. An electric distributioncable (3) is connected to the system, which extends to the subsealocated pipeline (4) which is to be heated. In a subsea location thereare arranged 3-to-2 phase transformers which connect electric power froma supply cable to sections of “piggyback” cables strapped onto theheated pipeline.

WO2006075913 describes a system for power supply to subseainstallations, comprising electric power supply cables for DEH of apipeline. The system is configurable to provide 3-phase power supply toan electric motor arranged subsea, when not heating the pipeline.

The Invention

According to the invention there is provided a subsea direct electricalheating assembly adapted to heat a hydrocarbon conducting steel(typically pipe-walls with ferromagnetic or similar material properties)pipeline arranged subsea. The assembly comprises a direct electricalheating cable (DEH cable) extending along and being connected to thesteel pipeline and a power transmission cable adapted to receiveelectric power from a power supply, arranged onshore or at surfaceoffshore, and to feed the direct electrical heating cable. According tothe invention the subsea direct electrical heating assembly furthercomprises a power conditioning arrangement arranged at a subsealocation, in a position between the power transmission cable and thedirect electrical heating cable, wherein the power transmission cableextends from the offshore or onshore power supply and down to the powerconditioning arrangement.

The power transmission cable should be understood to mean any cable orplurality of cables that transport electric power from an offshoretopside or onshore location to the subsea location of the powerconditioning arrangement. It should be understood that the powertransmission cable also could receive electric power via another subseaunit, such as a power distribution unit arranged subsea.

The power conditioning arrangement can advantageously comprise a subseacapacitor arrangement. Since the DEH cable combined with the pipelinethat shall be heated constitute an inductive load, the use of acapacitor arrangement will adapt delivered power to fit the load. Thatis, the power factor will be adjusted to balance the inductive load withthe power supply. As a result, the cross section of the powertransmission cable can be reduced compared to prior art solutions, inwhich power conditioning was performed onshore or on a floatinginstallation, far away from the load.

The power conditioning arrangement can also comprise a transformer. Thepower conditioning arrangement can also comprise a reactor.

The direct electrical heating cable is preferably arranged along andattached to the pipeline. A person skilled in the art knows this methodas the piggyback solution.

A piggyback solution can also be used for a subsea power cableindependent of DEH or EFH. I.e. a power cable can, during pipe laying orbefore trenching, be strapped to a hydrocarbon or produced-water orinjection-water transfer pipeline, to establish an electricalinterconnection between two offshore installations or between onshoreand offshore installations. For long interconnections of this type orsimilar ones without DEH a subsea reactor is suitable to overcome someof reactive power flow challenges associated with critical cable lengthsand transmission losses for high voltage ac-power cables.

In an embodiment according to the present invention, the subsea directelectric heating assembly is adapted to heat a plurality of pipelinesections which each constitutes a part of a longer pipeline. In thisembodiment the assembly comprises a plurality of DEH cables arrangedalong and/or in proximity to the pipeline sections. For each pipelinesection a said power conditioning arrangement is arranged between thepower transmission cable and the section heating cables associated toeach pipeline section.

In one embodiment, power from the power transmission cable is fed to adirect electrical heating cable adapted to heat a pipeline extendingbetween a subsea well and a subsea compression facility, through thesubsea power conditioning arrangement.

The subsea capacitor arrangement can be in the kV and kVAr range orabove. Preferably the capacitor arrangement comprises a capacitorelement arranged within a tank that prevents sea water entering the tankand getting into contact with the capacitor arrangement. The tank ispreferably pressure balanced and filled with a pressure compensationfluid.

On or off load tap-changer or tuning arrangements can be arranged incombination with a magnetic gear in order to enable operation withoutpenetration of a metallic water barrier of the subsea capacitor unit'stank or housing.

Also, the transformer is preferably arranged within the same tank.

The on or off load tap-changer or tuning arrangements can be adjusted byROV operations or an electric or a hydraulic actuator as typically usedfor subsea valve operations.

The capacitor arrangement is preferably a variable capacitorarrangement. The capacitance can then be adjustable between an upper andlower value, preferably by means of an actuator arranged within thetank. In this embodiment the operator is able to condition the deliveredpower to the load after installing the power conditioning arrangement,i.e. tuning of the DEH loops or enhanced power level control.

Correspondingly, the transformer can be an adjustable transformer fortuning of DEH loops or enhanced power level control.

One embodiment of an adjustable transformer is a transformer equippedwith an on or off load Lap-changer arrangement preferably capable of a+/−30% voltage control range or more.

A second embodiment of an adjustable transformer is a transformerequipped with an on load magnetic-field control arrangement preferablycapable of a +/−30% voltage control range or more, i.e. an adjustableair-gap or a Controllable Inductance Transformer.

Optionally one or more of the transformer out-put terminals can beequipped with series reactors that can be tapped or short circuited inorder to step the output current.

Alternatively to adjustable transformers, solutions with semiconductorbased power electronics can be used to limit the voltage applied on asection with DEH or EFH, i.e. typically thyristors in anti-parallel,transistors or other arrangements that can be operated in similarmanners as a soft-starter for continuous operation. The semiconductorscan be pressurized or located in a one atmosphere pressure controlledchamber associated with or within the pressure compensated transformertank/housing or the tank of the power conditioning arrangement.

The above methods for tuning of DEH loops or enhanced EFH power levelcontrol can generally be applied for power conditioning embodiments withsingle phase transformers, 3-to-2-phase transformers (typically Scott orLe Blanc connected) or 3-to-4-phase transformers, but some will bebetter suited than others for specific solutions.

In an advantageous embodiment the power transmission cable comprisesthree phases and three section heating cables are each connected betweentwo different pairs of phases of the power transmission cable. In onevariation of this embodiment, the assembly comprises three sets ofsection heating cables, wherein each set comprises two or more sectioncables. A section heating cable is a DEH cable adapted to heat apipeline section. This will be described below with reference to thedrawings.

Parallel pipelines or U-shaped return-pig-able flow lines or infieldlines could have parallel pipe sections with DEH applied separately withdedicated piggyback cables on each parallel pipe section powered via athree-to-two phase transformers or three-to-four phase transformers.

In an end-fed embodiment the power conditioning arrangement can beconnected between the power transmission cable and an end-fed pipelinesection. One transformer phase exits the tank through penetrator(s) andis connected to respective remote-end of said pipeline section.Furthermore, a second transformer terminal is connected to a sectionnear-end connection cable that connects to a near-end on the pipelinesection between said respective ends. The section near-end connectioncable is short-circuited to a steel structure of the power conditioningarrangement as is also the second transformer terminal. The steelstructure can for instance be the tank structure.

In a midpoint embodiment the power conditioning arrangement can beconnected between the power transmission cable and a midpoint fedpipeline section. Two transformer phases exit the tank throughpenetrators and are connected to respective ends of said pipelinesection. Furthermore, a third transformer terminal is connected to asection midpoint connection cable that connects to a midpoint on thepipeline section between said respective ends. The section midpointconnection cable is short-circuited to a steel structure of the powerconditioning arrangement as is also the third transformer terminal. Thesteel structure can for instance be the tank structure.

In one particular embodiment the power conditioning arrangement isconnected to a plurality of DEH cables which are arranged alongdifferent pipelines.

In another but somewhat similar embodiment the power conditioningarrangement is connected to a plurality of sets of a plurality of DEHcables, wherein each set is arranged to heat a plurality of separatepipelines.

In the embodiments according to the present invention, the powertransmission cable can extend for instance at least 30 km between thepower supply and said power conditioning arrangement.

With the term direct electric heating cable (DEH cable) is meant a cableprovided with alternating electric current in order to heat a subseapipeline adapted to carry hydrocarbons. In the art this comprisessolutions known as direct electric heating.

EXAMPLE OF EMBODIMENT

While the invention has been described in general terms above, a moredetailed example of embodiment will be given in the following withreference to the drawings, in which

FIG. 1 is a principle sketch of a subsea pipeline being heated with adirect electrical heating assembly which is powered from a floatingsurface installation;

FIG. 2 is a perspective view of a thermally insulated steel pipe havinga direct electrical heating cable and two power transmission cablesstrapped onto it;

FIG. 3 is a schematic drawing showing a setup from the prior art,showing a power supply arrangement arranged at a surface or onshorelocation;

FIG. 4 is a schematic drawing of the same features as shown in FIG. 3,however with a power transmission cable arranged between a subseacapacitor arrangement and the remaining components of the power supplyarrangement;

FIG. 5 is a schematic view of an end fed DEH assembly according to thepresent invention;

FIG. 6 is a schematic view of a midpoint fed DEH assembly according tothe present invention;

FIG. 7 is a schematic view of an end fed DEH assembly comprising aplurality of heating cable sections;

FIG. 8 is a schematic view of a midpoint fed DEH assembly comprising aplurality of heating cable sections;

FIG. 9 is a schematic view of a DEH assembly combining end feeding andmidpoint feeding;

FIG. 10 is schematic view of another DEH assembly combining end feedingand midpoint feeding;

FIG. 11 is a schematic view of a possible power conditioning arrangementbeing employed with a DEH assembly according to the invention;

FIG. 12 is a perspective view of the power conditioning arrangementshown in FIG. 11;

FIG. 13 is a principle perspective view of a variable capacitor elementin the power conditioning arrangement shown in FIG. 11;

FIG. 14 is a principle perspective view of the variable capacitorelement shown in FIG. 13 in an adjusted position;

FIG. 15 is a side view of the variable capacitor element shown in FIG.14;

FIG. 16 is a schematic view of an embodiment according to the invention;

FIG. 17 is schematic view of a DEH assembly according to a furtherembodiment of the present invention, without a capacitor arrangement;

FIG. 18 is a schematic view of a DEH assembly according to theinvention, wherein different pipes are heated with DEH cables which arefed from the same power conditioning arrangement; and

FIG. 19 is a schematic view of a DEH assembly according to theinvention, wherein a plurality of sets with parallel extending pipelinesare provided with DEH cables fed from a common power conditioningarrangement.

FIG. 1 shows a part of a hydrocarbon conducting pipeline 1 arranged onthe seabed. Along a section of the pipeline 1 is a direct electricalheating cable (DEH cable) 3. The DEH cable 3 connects to the saidsection of the pipeline 1 in two locations and provides that alternatingelectric current flows through the steel of the pipeline 1, between thesaid locations. At the locations of electric contact between the DEHcable 3 and the steel of the pipeline 1, there is also contact to theambient sea water. Thus, some current will flow through the sea water,along the pipeline.

Between the DEH cable 3 and a power supply arranged on a floatinginstallation 5 extends a power transmission cable 7. It is also known toprovide power through a power transmission cable 7 from an onshorelocation.

FIG. 2 is a perspective cutaway view of the pipeline 1. Onto thepipeline 1 there are strapped one DEH cable 3 and two power transmissioncables 7. This technique is known in the art as piggyback cabling. Itshould be noted that the power transmission cables 7 shown strapped ontothe pipeline 1 in FIG. 2 are not necessarily used to feed power to theDEH cable 3. I.e. they may be used to feed other DEH cables than the oneshown, or to feed other subsea equipment.

On the steel section of the pipeline 1 there is arranged thermalinsulation. This reduces the heat loss to the ambient sea water when thesteel is heated.

FIG. 3 is a schematic drawing showing a setup from the prior art, namelythe patent application publication EP2166637. The drawing shows a powersupply arrangement adapted to provide electric current to a DEH cablearranged subsea, such as the DEH cable 3.

FIG. 4 is a modification of the drawing shown in FIG. 3, according to anembodiment of the present invention. In this embodiment, the capacitorarrangement which is arranged before the DEH cable 3, is arranged at asubsea location, close to the DEH cable 3. As a result of this, a powertransmission cable 7 is arranged between the DEH cable 3 and the otherparts of the power supply. As illustrated in FIG. 1, the powertransmission cable 7 extends from a surface location (or an onshorelocation) down to the DEH cable 3.

FIG. 5 and FIG. 6 show two types of setup for a DEH assembly accordingto the present invention. In these embodiments, as well as foradditional embodiments to be described later with reference toadditional drawings, it is assumed a carbon steel pipeline of 30″, andpower transmission cables of 52 kV. It should however be clear to theperson skilled in the art that the invention is not limited to theseconstraints. Thus the pipeline diameter may be smaller or larger, andpower transmission cables of higher or lower voltage may be employed,for instance 132 kV. In 2011 the upper limit for electrical subseaconnectors or penetrators recognized by the industry was 132 (145) kV,ref. Mecon DM 145 kV. Furthermore, the embodiments described herein arenot restricted to use at deep waters, such as 1000 to 2000 meters.However the described embodiments according to the invention are wellsuited for such depths.

In the embodiment shown in FIG. 5, approximately 50 km of thermallyinsulated pipeline 1 is heated with a DEH assembly according to theinvention. From a not shown power supply, which for instance can bearranged on a floating installation or an onshore facility, electricpower is supplied through a power transmission cable 7. The powertransmission cable 7 has three separate conductors or phases (asindicated with the three tilted lines schematically crossing the powertransmission cable 7).

The three phase power transmission cable 7 connects to a powerconditioning arrangement 100. In this embodiment, the power conditioningarrangement 100 comprises a capacitor arrangement 110 and a transformer120. To the power conditioning arrangement 100 a DEH cable 3 isconnected, which extends along the pipeline 1. The electric powerdelivered by the power transmission cable 7 can be modified and/orcompensated at the subsea location to fit the inductive load of the DEHcable 3 (i.e. the DEH cable and the connected pipeline). That is, inthis embodiment the delivered power from the power transmission cable 7is, in the power conditioning arrangement 100, transformed as a singlephase load where the voltage level is decreased (current is increased)and the power factor (cos φ) is adapted to suit an inductive load.

Still referring to FIG. 5, from the power conditioning arrangement 100 ajumper connects to a first connection point 9 to the pipeline 1 (lefthand side of FIG. 5). At the opposite end of the pipeline 1 section inquestion, the DEH cable 3 connects to a second connection point 9, 50 kmaway. The connection points 9 are arranged in a current transfer zone 11(CTZ), provided with anodes 13. Between the current transfer zones 11,there are also arranged intermediate anodes 15 for cathodic protectionof the pipeline, particularly in case of cracks in the coating/thermalinsulation. The intermediate anodes 15 also function as earth points forthe pipe. The embodiment shown in FIG. 5 is referred to as an end pointfed system, in which the two single-phase terminals are connected to thetwo opposite ends of a pipe section.

FIG. 6 schematically illustrates another embodiment of the presentinvention. In this embodiment the midpoint fed system is employed. Inthis embodiment, two phases are used, one connected to respective endsof a pipeline section of approximately 100 km. The length of thepipeline 1 which is heated with the two phases is thus twice the lengthheated in the embodiment shown in FIG. 5 (employing the end point fedsystem). Although not shown in FIG. 6, one could also connect the pointin between the two distant connection points 9 to earth (a thirdconductor to the pipeline midpoint from capacitors on the transformer).

As shown in FIG. 6, two DEH cables 3 extend out from the powerconditioning arrangement 100. The DEH cables 3 extend in oppositedirections along the pipeline 1 which is to be heated by the DEHassembly. Corresponding to the features of the embodiment shown in FIG.5, the DEH cables 3 connect to respective connection points 9 (100 kmapart) arranged within a current transfer zone 11.

In this embodiment, as shown in FIG. 6, the power conditioningarrangement 100 converts the three phases in the power transmissioncable 7 into two phases, of which one is applied on each of therespective DEH cables 3.

In the embodiments shown in FIG. 5 and FIG. 6, the capacitor arrangement110 will adapt the electric power delivered to the DEH cable(s) 3, asthe DEH cable(s) 3, together with the pipeline 1 which shall be heatedconstitute an inductive load. As a result, less current flows in thepower transmission cable 7 and hence a smaller cable with less conductor(copper) cross section can be installed. The needed conductor crosssection may be reduced to approximately ½ to ¼ of the cross section ofthe similar prior art solutions without the subsea capacitor arrangement110.

FIG. 7 and FIG. 8 schematically show a DEH layout where the pipeline 1is divided into three heated pipeline sections 1 a. In both embodimentselectric power is delivered through a 52 kV power transmission cable 7.In the embodiment shown in FIG. 7, a (not indicated) DEH cable 3 extendsbetween two connection points 9 on each side of each of the threepipeline sections 1 a. Between each of the three DEH cables 3 and thepower transmission cable 7 there is connected a power conditioningarrangement 100 comprising a capacitor arrangement 110 (cf. FIG. 5). Inthis embodiment, each pipeline section 1 a is approximately 50 km long.Thus the illustrated DEH assembly heats a pipeline 1 length ofapproximately 150 km.

The embodiment shown in FIG. 8 is similar to the one shown in FIG. 7,however a midpoint fed system is employed, such as the one describedwith reference to FIG. 6 above. Also in this embodiment exhibits threepipeline sections 1 a, however since the midpoint fed system is employedeach pipeline section 1 a can be made longer, such as for instance 50 to100 km long. Each pipeline section 1 a and associated power conditioningarrangement 100 can correspond to the embodiment shown in FIG. 6.

FIG. 9 shows another embodiment of a DEH assembly according to thepresent invention. In this embodiment, two pipe sections 1 a of 80 kmare heated with the midpoint fed system, whereas a third pipe section 1a of 40 km is heated with the endpoint fed system. The endpoint fed pipesection 1 a of 40 km is close to a power supply and may be partiallyabove the sea surface. Hence there is no power conditioning arrangement100 between the typically two-core power transmission cable 7 and theDEH cable 3, associated to this pipeline section 1 a. As the pipeline 1continues a long distance along the seabed, such as to a subseahydrocarbon well (not shown) the other two pipeline sections are heatedwith the DEH assembly according to the present invention. Between thethree phase power transmission cable 7 and the DEH cables 3 there arearranged, in the subsea location close to the pipeline 1, a powerconditioning arrangement 100. In this embodiment, the power conditioningarrangement 100 comprises a three-to-two phase transformer 120. It alsocomprises a capacitor arrangement 110 with a capacitor element 115arranged between a section midpoint connection 4 to the pipeline at themid point between the connection points 9 of the respective pipelinesection 1 a, and the transformer 120. The transformer 120 providesgalvanic segregation between the primary side supplied via thethree-phase power transmission cable 7 and the secondary side that iselectrically connected to pipeline via the DEH cable 3 and the midpointconnection 4.

As will be explained later, with reference to FIG. 16, the sectionmidpoint connection 4 between the said pipeline section midpoint and thetransformer 120, may be connected to the chassis or the outer tank/shellof the transformer 120.

FIG. 10 shows a particular embodiment exhibiting three approximatelyequally long pipeline sections 1 a of 60 km, and a shorter pipelinesection of about 20 km. As with the embodiment shown in FIG. 9, aseparate short typically two-core power transmission cable 7 extend froman onshore power supply to the short pipeline section 1 a of 20 km. Forthis pipeline section 1 a there is no power conditioning arrangement 100arranged subsea or between the power transmission cable 7 and the DEHcable 3. In association with each of the subsequent three pipelinesections 1 a there is however arranged a power conditioning arrangement100. Furthermore, in this embodiment there is not arranged any sectionmidpoint connection 4 between the transformer 120 and the pipeline 1. Inthis embodiment, the transformer 120 is a single phase transformer (i.e.a single phase transformer 120 for each power conditioning arrangement100). The transformer 120 provides galvanic segregation between theprimary side supplied via the three-phase power transmission cable 7 andthe secondary side that is electrically connected to pipe-line via theDEH cable 3.

In the embodiment illustrated in FIG. 10, the DEH assembly associatedwith the three longest pipeline sections 1 a is coupled to a unique pairof two phases of the three phase power transmission cable 7. That is,the three respective transformers 120 associated with the three long (60km) pipeline sections 1 a are connected to transmission cable phaseL1+L3, L2+L3, and L1+L2, respectively. Between each transformer 120 andDEH-cable 3, there is coupled a capacitor arrangement 110. With suchcoupling layout, one achieves a balanced load on the phases L1, L2, L3of the power transmission cable 7 when the length or load of each pipesection 1 a is the same.

FIG. 11 shows a schematic view of a power conditioning arrangement 100,adapted to be installed in a subsea environment. The power conditioningarrangement 100 has a capacitor arrangement 110 arranged within a rigidtank 105. The tank 5 is filled with a liquid, such as an oil. Thecapacitor arrangement 110 can also have arranged a transformerarrangement 120 within the same tank 105. Electrically connected to thecapacitor arrangement 110 and/or the transformer arrangement 120 is apair of electric cables 103 which connect to a pair of penetrators 130.The electric cables 103 may be connected to the capacitor arrangement110 by connection to the penetrators 130 in a subsea environment. Thepower conditioning arrangement 100 can thus be added to an existingelectric system subsea and/or may be disconnected for maintenance orreplacement. The electric cables 103 may be connected to the DEHcable(s) 3, or may indeed be the DEH cable(s) 3 itself.

In order to make the subsea power conditioning arrangement 100 suitablefor installation in a subsea environment, possibly with large ambientpressures, the liquid within the tank 105 is pressure balanced. Thepressure balancing is provided with a pressure balancing section 135.The pressure balancing section 135 is functionally connected to theinterior of the tank 5 through a pressure balance liquid line 140.

The pressure balance liquid line 140 extends between the interior of thetank 105 and a main metal bellows 145 which can be filled with oil. Themain bellows 145 is compressible. Thus when the power conditioningarrangement 100 is lowered into the sea, the ambient pressure willcompress the main bellows 145. This results in approximately the samepressure within the main bellows 145 and the tank 105 as the ambientwater pressure. In order to provide a slightly larger pressure withinthe main bellows 145 and the tank 105, a weight 150 is arranged on themain bellows 17 in such way that it preloads or compresses the bellows145. Thus the pressure in the tank 105 will always be slightly higherthan the pressure of the ambient water. This prevents leakage of seawater into the tank 105. In order to render it possible to fill ordischarge liquid into or out of the main bellows 145 (such as with anROV), a connection line and valve 147 is arranged in association to themain bellows 145.

Outside the main bellows 145 there can be arranged an auxiliary bellows155. The auxiliary bellows 155 encloses the main bellows 145 togetherwith a bottom plate. The auxiliary bellows 155, i.e. the volume betweenthe auxiliary bellows 155 and the main bellows 145 can also be filledwith oil or another appropriate barrier liquid. In this way the mainbellows 145 is protected from sea water. Corresponding to the mainbellows 145, the auxiliary bellows 155 is also provided with aconnection line and valve 157. In addition the auxiliary bellows 155 isprovided with an indication pin 159 extending upwards from the top ofthe auxiliary bellows 155. The indication pin 159 indicates the verticalposition of the top of the auxiliary bellows 155 and thus tells theoperator if liquid amount in the auxiliary bellows 155 needs to beincreased or decreased.

As will be appreciated by the person skilled in the art, the pressurebalance function is provided also without the auxiliary bellows 155.Also, without the auxiliary bellows 155, the indication pin 159 could bearranged on the main bellows 145.

Surrounding the main bellows 145 and the auxiliary bellows 155 is arigid enclosure 160 which protects the bellows 145, 155, such as fromimpacts from falling objects or collision with an ROV.

When employing a power conditioning arrangement 100 in the variousembodiments according to the present invention, one may arrange both acapacitor arrangement 110 and a transformer arrangement 120 within thesame tank 105. One may also arrange them in separate tanks. However onewould then have to connect them together with electric jumpers andadditional wet-mate connectors. According to the present invention,there may also be embodiments without transformers (cf. FIG. 17).

FIG. 12 shows a more realistic perspective view of the subsea powerconditioning arrangement 100. In this illustration the pressurebalancing section 135 also comprises some bladder compensators 165.These are not present in the embodiment shown in FIG. 11. The bladdercompensators 165 are connected to the auxiliary bladder 155 in stead ofthe connection line and valve 157 shown in FIG. 11. Each bladdercompensator 165 has a rigid vessel holding a gas volume and a liquidvolume, wherein the volumes are separated with a flexible bladder. Theliquid line (not shown) extending between the bladder compensators 165and the interior of the auxiliary bellows 155 can have a valve adaptedfor filling and/or discharging liquid (e.g. oil) into or out of thebladder compensators 165 and the auxiliary bellows 155.

It is now referred to the drawings of FIG. 13, FIG. 14, and FIG. 15.These drawings show principle sketches of a possible variable capacitorarrangement 110. The capacitor arrangement 110 comprises a set of firstplates 111 and a set of second plates 113. As is not shown but will beappreciated to the person skilled in the art, the set of first plates111 are functionally connected to one of the electric cables 103 whereasthe set of second plates 113 are functionally connected to the otherelectric cable 103 (cf. FIG. 11). Furthermore, the set of second plates113 is connected to a pivot rod 115 which is adapted to be pivoted bymeans of an electric actuator (not shown) within the tank 105. When theset of second plates 113 is pivoted with respect to the stationary setof first plates 111, the capacitance varies.

FIG. 13 shows a situation wherein the first plates 111 are aligned withthe second plates 113. FIG. 14 shows a situation wherein the secondplates 113 have been rotated about 90 degrees with respect to thealigned position shown in FIG. 13. In this position the overlapping areaof the first and second plates is less than in the aligned position,thereby reducing the capacitance of the capacitor arrangement 110. Withadditional rotation of the set of second plates 113, they can be movedinto a position in which substantially no overlapping exists between thefirst and second plates 111, 113. The capacitance of the capacitorarrangement can then be practically zero. FIG. 15 shows the samesituation as in FIG. 14, in a side view.

In a more realistic embodiment, the capacitor arrangement 110 will havemore plates 111, 113 and the plates can be arranged closer to eachother. Furthermore, in stead of having one capacitor element as shown inFIG. 13, the capacitor arrangement 110 can comprise a plurality ofcapacitor elements, that is a plurality of the assemblies shown in FIG.13. These can be connected in parallel and some or all of them may be ofthe variable type. The gaps between the plates 111, 113 can be filledwith the liquid, preferably oil, present in the tank 105.

FIG. 16 schematically shows the power conditioning arrangement 100 inassociation to a midpoint fed pipeline 1 or pipeline section 1 a. Inthis embodiment, the power conditioning arrangement 100 comprises atransformer 120 and capacitor arrangement 110. Two of the phases outfrom the transformer 120 are connected in parallel with the capacitorarrangement 110. After the capacitor arrangement 110, the two phasesexit the tank 105 through the penetrators 130. One of the phases isconnected to one end of the pipeline section 1 a and is terminated tothe pipeline 1. In this embodiment, the cable 103 exiting from thepenetrator 130 is the same cable as the DEH cable 3 which is piggybackedonto the pipeline 1. The second phase is connected to the other end ofthe pipeline section 1 b and is terminated to the pipeline 1. The thirdphase exiting the transformer 120 is functionally connected to thesection midpoint connection cable 4, which connects to the midpoint ofthe pipeline section 1 b.

In order to reduce the amount of penetrators and thereby the cost andcomplexity, the section midpoint connection 4 cable connected to thepipeline section 1 b is short circuited at the steel structure of thepower conditioning arrangement 100, such as on the exterior face of thetank 105. This can be done in different ways. For example by connectingthe section midpoint connection 4 cable to a steel sleeve and thenwelding this steel sleeve to the steel structure of the tank 105. On theinside of the tank 105, the third phase can then be connected to thetransformer 120 with a copper cable that is short circuited to the innerside of the tank 105. By doing this there is no need for a cable goingthrough the capacitor assembly and therefore one less penetrator isneeded.

As will be appreciated by the person skilled in the art, the powerconditioning arrangement 100 is connected to a not shown powertransmission cable 7, as shown in the above described embodiments.

FIG. 17 shows an additional embodiment of a subsea DEH assemblyaccording to the invention. The embodiment corresponds in many respectsto the embodiment described with reference to FIG. 10. However, in theembodiment shown in FIG. 17 the power conditioning arrangement 100 doesnot comprise a transformer, hence the various sections with DEH do nothave galvanic segregation. For the adjacent systems galvanic segregationis provided by the feeding transformers and optionally the receivingtransformer in the far-end if installed.

FIG. 18 shows a particular embodiment according to the presentinvention. On the seabed there are arranged a plurality of differentpipelines 3. Each pipeline is arranged with a DEH cable 3. In thisembodiment, each pipeline is heated with an endpoint fed system, whereineach respective DEH cable 3 is fed with a common power conditioningarrangement 100. As with the embodiments above, the power conditioningarrangement 100, which is arranged subsea, receives power through apower transmission cable 7.

FIG. 19 is an embodiment similar to the embodiment shown with referenceto FIG. 18. However, in the embodiment shown in FIG. 19 each DEH cable 3is arranged in a configuration to heat a plurality (three) of pipelines1. That is, each DEH cable 3 is associated with three pipeline segmentsthat extend between the same locations. Moreover, with the embodimentshown in FIG. 19, one power conditioning arrangement 100 provides powerto three sets of three DEH cables 3. As will be understood by the personskilled in the art, with the setup shown in FIG. 19 it will bebeneficial to have the pipelines 1 close to each other in order toreduce the necessary length of the DEH cables 3 and the jumpersconnecting each pipeline (or each pipeline segment 1 a of differentpipelines 1, respectively).

The person skilled in the art will appreciate that the present inventionis suited for other embodiments than the ones shown above, such as thepipe-in-pipe technique which is assumed known to the skilled person.

The above described embodiments can typically be employed with pipelineshaving a diameter in the range of e.g. 20″ to 30″ and with a length offor instance more than 100 km. As shown by dividing the heated pipeline1 into sections 1 a, a pipeline which is much longer than 100 km can beheated.

To illustrate the technical advantages brought about with the presentinvention, the following example is given. When using the directelectric heating assembly according to the present invention, one canfor instance eliminate 2-10 DEH risers (cf. power transmission cable 7in FIG. 1) extending down from a floating platform (typically for fieldswith 2-10 heated flowlines), where each riser typically comprises twoconductors with a copper cross section of 1200-1600 mm². All theserisers can be replaced with one 3 core riser having three conductorswith 200 mm² to 800 mm².

1. A subsea direct electrical heating assembly adapted to heat ahydrocarbon conducting steel pipeline arranged subsea, the subsea directelectric heating assembly comprising: a direct electrical heating cableextending along and being connected to the steel pipeline and a powertransmission cable receiving electric power from a power supply,arranged onshore or at surface offshore, and feeding the directelectrical heating cable; a power conditioning arrangement arranged at asubsea location, in a position between the power transmission cable andthe direct electrical heating cable; and wherein the power transmissioncable extends from the offshore or onshore power supply and down to thepower conditioning arrangement.
 2. A subsea direct electrical heatingassembly according to claim 1, wherein the power conditioningarrangement comprises a subsea capacitor arrangement.
 3. A subsea directelectrical heating assembly according to claim 1, wherein the powerconditioning arrangement comprises a transformer.
 4. A subsea directelectrical heating assembly according to claim 1, wherein the directelectrical heating cable is arranged along and attached to the pipeline.5. A subsea direct electrical heating assembly according to claim 1,wherein the subsea direct electrical heating assembly is adapted to heata plurality of pipeline sections which each constitutes a part of alonger pipeline as the direct electric heating assembly comprises aplurality of direct electric heating cables arranged along and/or inproximity to the pipeline sections and that for each pipeline section asaid power conditioning arrangement is arranged between the powertransmission cable and the section heating cables associated to eachpipeline section.
 6. A subsea direct electrical heating assemblyaccording to claim 1, wherein power from said power transmission cableis fed to a direct electrical heating cable adapted to heat a pipelineextending between a subsea well and a compression facility, through thesubsea power conditioning arrangement.
 7. A subsea direct electricalheating assembly according to claim 2, wherein the subsea capacitorarrangement is in the kV and kVAr range or above, comprising a capacitorelement arranged within a tank that prevents sea water entering thetank, wherein the tank is pressure balanced and filled with a pressurecompensation fluid.
 8. A subsea direct electrical heating assemblyaccording to claim 3, wherein the transformer is arranged within thetank.
 9. A subsea direct electrical heating assembly according to claim7, wherein the capacitor arrangement is a variable capacitorarrangement, the capacitance of which is adjustable between an upper andlower value by means of an actuator arranged within the tank.
 10. Asubsea direct electrical heating assembly according to claim 3, whereinthe transformer is an adjustable transformer.
 11. A subsea directelectric heating assembly according to claim 3, wherein the powertransmission cable comprises three phases and that three section heatingcables are each connected between a different pair of phases.
 12. Asubsea direct electric heating assembly according to claim 11,comprising three sets of section heating cables, wherein each setcomprises two or more section cables.
 13. A subsea direct electricheating assembly according to claim 8, wherein the power conditioningarrangement is connected between the power transmission cable and amidpoint fed pipeline section, wherein two transformer terminals exitthe tank through penetrators and are connected to respective ends ofsaid pipeline section, and that a third transformer terminal isconnected to a section midpoint connection cable that connects to amidpoint on the pipeline section between said respective ends, whereinthe section midpoint connection cable is short circuited to a steelstructure of the power conditioning arrangement as is also the thirdtransformer terminal.
 14. A subsea direct electric heating assemblyaccording to claim 1, wherein the power transmission cable extends atleast 30 km between the power supply and said power conditioningarrangement.
 15. A subsea direct electric heating assembly according toclaim 1, wherein the power conditioning arrangement is connected to aplurality of DEH cables which are arranged along different pipelines.16. A subsea direct electric heating assembly according to claim 1,wherein the power conditioning arrangement is connected to a pluralityof sets of a plurality of DEH cables, wherein each set is arranged toheat a plurality of parallel pipelines.